NOx emissions from combustion engines (for instance diesel engines) with an exhaust gas catalyst can be reduced by injection of reducing components into the exhaust pipe of these diesel engines before the exhaust gas is contacted with a SCR or an EGR catalyst. The use of a urea-comprising aqueous stream as a reducing agent for this purpose offers special advantages, in particular in diesel engines used to power cars and trucks. The quality requirements to be met by such a urea-comprising aqueous stream are known, for example from DIN Vornorm V70070.
Such a urea-comprising aqueous stream is currently prepared by dissolving commercially available solid urea in clean water while adding heat. The solid urea is available in the form of, for example, urea granulate or urea prills.
A problem encountered is that commercially available solid urea is contaminated with formaldehyde or other additives, because the formaldehyde or the other additives are added to improve the properties of the solid urea and facilitate granulation or prilling of the urea. According to the Vornorm cited above only very low amounts of contaminants are allowed in a urea-comprising aqueous stream that will be used in a unit for the reduction of NOx. A urea-comprising aqueous stream obtained by dissolving solid urea does not meet the quality requirements according to the Vornorm.
At this moment the solid urea or the urea-comprising aqueous stream obtained from it is purified by removing the formaldehyde, the reaction products of formaldehyde and urea and other additives using known separation techniques. However, this process is expensive.
It is the aim of the invention to eliminate these drawbacks.
This is achieved by separating the urea-comprising aqueous stream directly from or after a recovery section in a urea production process and thereafter dilute the urea-comprising aqueous stream with water until the urea-comprising stream comprises 30-35 wt % urea.
In this way a urea-comprising aqueous stream that does not contain formaldehyde, reaction products of formaldehyde and urea or other additives is directly obtained.
A further advantage is that no solid urea needs to be dissolved in water in order to obtain a urea-comprising aqueous stream.